Simple circuiting question, using Arduino

Here's a short Raspberry Pi discussion about driving a 5V relay from a RasPi GPIO.  The second comment shows a simple NPN transistor circuit.  I believe you can use the same circuit with your 24V external device provided that you select a transistor with a high voltage rating.  [This opinion provided without warranty.]

Note that the circuit has a 1K current limiting resistor between the GPIO and transistor base.  You don't want to drive an NPN transistor base directly from a GPIO with the transistor's emitter grounded, because the voltage drop across the transistor base-emitter junction should not be more than 0.6 volts and if the GPIO pin tries to pull it up to 3.3V you're likely to damage something.  The 1K resistor limits the current to about 2.7mA assuming a 3.3V GPIO, or about 4.4mA with a 5V GPIO.

The transistor multiplies current by a factor of "beta", with is typically 10-20.  That is, the transistor can amplify a 2.7mA current through its base-emitter junction to 27 mA or more through its collector-emitter junction depending on what's hooked up.  If this is not enough current, you can use two NPN transistors in the form of a Darlington Pair, which gives you a current multiplication of "beta" squared.

Before using your external 24V device, be sure to test using an LED so you can tell if you've got things hooked up correctly.  You typically want to limit LED current to 2-5mA, which means you need a 5-10K resistor between the LED and 24V.  The voltage drop across a working LED is 2.4V or so, depending on color.  If you put more than that across an LED, it will emit a very bright light once before it turns into a DED (dark-emitting diode).

Edit: a high-voltage transistor with at least 40V reverse breakdown voltage should be all the protection you need.  However, since 24V will easily kill a micro-controller, if you're worried you can use a 5V relay or opto-isolator to control the 24V circuit.

Here's a short Raspberry Pi discussion about driving a 5V relay from a RasPi GPIO.  The second comment shows a simple NPN transistor circuit.  I believe you can use the same circuit with your 24V external device provided that you select a transistor with a high voltage rating.  [This opinion provided without warranty.]

Note that the circuit has a 1K current limiting resistor between the GPIO and transistor base.  You don't want to drive an NPN transistor base directly from a GPIO with the transistor's emitter grounded, because the voltage drop across the transistor base-emitter junction should not be more than 0.6 volts and if the GPIO pin tries to pull it up to 3.3V you're likely to damage something.  The 1K resistor limits the current to about 2.7mA assuming a 3.3V GPIO, or about 4.4mA with a 5V GPIO.

The transistor multiplies current by a factor of "beta", with is typically 10-20.  That is, the transistor can amplify a 2.7mA current through its base-emitter junction to 27 mA or more through its collector-emitter junction depending on what's hooked up.  If this is not enough current, you can use two NPN transistors in the form of a Darlington Pair, which gives you a current multiplication of "beta" squared.

Before using your external 24V device, be sure to test using an LED so you can tell if you've got things hooked up correctly.  You typically want to limit LED current to 2-5mA, which means you need a 5-10K resistor between the LED and 24V.  The voltage drop across a working LED is 2.4V or so, depending on color.  If you put more than that across an LED, it will emit a very bright light once before it turns into a DED (dark-emitting diode).

Edit: a high-voltage transistor with at least 40V reverse breakdown voltage should be all the protection you need.  However, since 24V will easily kill a micro-controller, if you're worried you can use a 5V relay or opto-isolator to control the 24V circuit.